Content ID is an ID consisting only of numbers that is assigned when submitting the contents.
It is not necessary to keep a special record, but you may be required to enter the ID when using the image response function.
The number is displayed at the end of the URL of the content.
Illustration: https://www.pixiv.net/artworks/○○○○○○○○
Novel: http://www.pixiv.net/novel/show.php?id=○○○○○○○
*The part you see in place of ○○○ ○○○○ will be the Content ID